Alien Invasion. By David Williams.

The  10 Worst Invaders in the Pacific Northwest

Cheatgrass { Bromus tectorum }
Arrived in country in late 1800s in contaminated wheat. Thrived because it could grow on disturbed habitat, preadapted to wet, cold winters and dry, hot summers, and because it outcompeted natives after fire.

English ivy { Hedera helix }
Spreads aggressively by seed and vegetatively. Introduced for erosion control as early as the 1860s. Planted in Seattle city parks in 1890s.

European beach grass { Ammophila arenaria }
Introduced in the late 1800s to stabilize dunes around San Francisco. It thrives on burial, which is necessary for vigorous growth and flowering, and becomes dominant by heavy foliage cover.

Iberian/Purple/Yellow Starthistle { Centaurea iberica, C. calcitrapa, C. solsitialis }
Copious production (150,000 seeds/plant) of wind blown seeds that spread by vehicles, animals (particularly finches), and contaminated seed. Arrived as a contaminant, probably, as early as 1920s in eastern Washington.

Japanese knotweed { Polygonum cuspidatum }
Introduced to the U.S. for use in ornamental hedges and for erosion control. Plants emerge early in spring, which helps them get established and dominate a site. A deep, thick mat of rhizomes prevents easy root removal.

Leafy Spurge { Euphorbia esula }
High germination rate, explosive seeds, and vegetative reproduction. Unknown arrival date, but probably in contaminated seed or ship ballast.

Purple loosestrife { Lythrum salicaria }
Spreads both vegetatively and by seed to form dense, monotypic stands of little or no habitat benefit. First found on the shores of Lake Washington in 1929. Popular horticulturally and medicinally.

Reed canary grass { Phalaris arundinacea }
Aggressively spreads by rhizomes, out-competing and eliminating natives. May be a native that has crossed with non-natives to produce more vigorous growth.

Scotch broom { Cytisus scoparius }
Tolerant to a wide variety of soil types with long lived seeds and the ability to fix nitrogen. Seeds were given as gift from English consul in Hawaii to a Captain Grant in 1850 on Vancouver Island.

Tamarisk { Tamarix ramosissma }
A single plant can produce a half million seeds per year. Stems and leaves secrete salt, which inhibits growth of other plants. Introduced for erosion control and as ornamental in California in 1861.

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